1. Field of the Application
The present invention relates to powder metallurgy and, more particularly, to methods and apparatus for producing globular powders or granules from meltable materials which may be both metallic and nonmetallic.
2. Description of the Prior Art
There are known methods and apparatus for producing granules of metals and alloys by melting the end of a fast-rotating rod-type billet with the use of an electric heat source, for example, an electron beam. The centrifugal forces spray the molten metal inside a sealed chamber so that the molten metal solidifies into spherical particles whose size is determined by the density of the metal, the diameter and the rotation speed of the billet (cf. Italian Patent No. 712,814, Cl. B22 d, of 1966 and Federal Republic of Germany Pat. No. 1,291,842, Cl. 21h 16/60, of 1966).
In the United States, a common type of an apparatus for producing metal granules is the one that comprises a collet means for clamping a cylinder-shaped billet to be melted, a mechanism for rotating the billet and moving it in the longitudinal direction, and a melting chamber filled with an inert gas and accommodating a nonconsumble electrode; finally, the apparatus comprises wiring and busbar means to connect the collet means and the nonconsumable electrode to a power source (cf. U.S. Pat. No. 3,099,041, Cl. 425-8 of 1963). An electric arc is produced between the nonconsumable electrode and the end of the rotating billet. The arc melts the billet's end; the molten metal is sprayed by the centrifugal forces and particles of metal solidify in the inert gas inside the melting chamber.
The foregoing devices and apparatus all have a limited output, the main reason for this being that it is extremely difficult to introduce a new billet into the melting chamber without unsealing that chamber. In addition, the billet diameter is limited, being determined by the relatively small area of the heat-affected zone. With a cross-sectional area of the billet several times greater than the area of the heat-affected zone, the end of the billet cannot be heated uniformly. As a result, only the center of the billet is melted and sprayed, whereas at the periphery the metal only reaches the plastic state to be torn by the centrifugal forces into large shapeless lumps. The output of good granules is thus sharply reduced.
The closest prototype of the present invention, with regard to its technical features and positive effect, is the granulation device developed in the Soviet Union and patented in a number of countries (cf. U.S. Pat. No. 3,752,610, Cl. 425-6, of 1973).
The latter device comprises a sealed round chamber, its axis extending horizontally. On one side, the chamber is provided with a round opening for a billet, which opening is coaxial with the chamber. On the other side, the chamber is provided with a lid into which there is built a heat source. The latter is a nonconsumable tungsten electrode arranged coaxially with the chamber. The device further includes a billet rotating mechanism, composed of two synchronously rotating horizontal rolls and arranged outside the chamber. The axes of the rolls are parallel to the axis of the chamber. Arranged above the rolls is a screw-type mechanism intended to drive a billet between the rolls and through the round hole into the chamber. The apparatus further includes a charging means composed of a tray and a revolving drum, a brush-type lead to the rotating billet, a power source to produce an electric arc between the rotating billet and the nonconsumable electrode, and other mechanisms and components.
A batch of billets is loaded in the charging means. The chamber is evacuated and filled with an inert gas. The billets are fed one by one to the melting zone of the chamber to be melted by the electric arc produced between the nonconsumable electrode and the rotating billet. The centrifugal forces break the molten metal into small particles which crystallize as they fly from the billet to the lateral walls of the chamber. The granules thus produced accumulate at the bottom of the chamber; the unmelted part of the billet is also dropped to the bottom. Unlike the American apparatus described above (cf. U.S. Pat. No. 3,099,041, Cl. 425-8), the apparatus under review is quite economical because the entire contents of the charging means can be processed without unsealing the chamber. That notwithstanding, this advanced apparatus (cf. U.S. Pat. No. 3,752,610, Cl. 425-6) is not free from some of the disadvantages typical of all conventional devices, such as a relatively limited output or limitations imposed on billet diameters.
Furthermore, the coaxial arrangement of the nonconsumable electrode and the rotating billet makes maintenance difficult. For example, in order to push the unmelted part of a billet through the round opening into the chamber, one must first use a special mechanism to move aside the heat source. It is difficult to observe the granulation process because the sight glasses are arranged at an angle to the axis of the rotating billet.
Besides, some of the brush material, which is normally a rapidly wearing material, such as graphite or copper, is caught by the granules, which affects the purity of the product.
Finally, none of the above apparatus, wherein an electric arc or electronic beam are used to melt a rotating billet, can be used to granulate non-conducting materials.